1. Field of the Invention
This invention relates to multijunctions in a body of a semiconductor material and a temperature gradient zone melting process and diffusion heat treatment for making the same.
2. Description of Prior Art
W. G. Pfann describes in his book "Zone Melting" John Wiley & Sons, Inc. (1966), a thermal gradient zone melting process to produce P-N junctions within the bulk of a semiconductor material. In his method, either sheets or wires of a suitable metal-rich liquid are moved through a semiconductor material in a thermal gradient. Doped liquid epitaxial material is left behind the migrating liquid zone. For two decades, this process of temperature gradient zone melting has been practiced in an attempt to make a variety of semiconductor devices.
In our copending applications -- "Method of Making Deep Diode Devices", U.S. Pat. No. 3,901,736; "Deep Diode Device Production and Method", Ser. No. 411,021; "Deep Diode Devices and Method and Apparatus", Ser. No. 411,001, now abandoned in favor of Ser. No. 552,154; "High Velocity Thermomigration Method of Making Deep Diodes", U.S. Pat. No. 3,898,106; "Deep Diode Device Having Dislocation-Free P-N Junctions and Method", U.S. Pat. No. 3,902,925; and "The Stabilized Droplet Method of Making Deep Diodes Having Uniform Electrical Properties", U.S. Pat. No. 3,898,361; and "Thermomigration of Metal-Rich Liquid Wires Through Semiconductor Materials", Ser. No. 411,018, and now U.S. Pat. No. 3,898,362; assigned to the same assignee of this application, we teach the stability of droplets for thermomigration, and preferred planar orientations as well as critical dimensions of the droplets affecting the migration thereof.
In all of Pfann's work and the above copending applications, no more than one P-N junction in the case of a single droplet liquid zone or single sheet liquid zone temperature gradient zone melting process, and no more than two P-N junctions in the case of a single wire liquid zone temperature gradient zone melting process can be produced. In other words, in the prior art, no more than one type of conductivity and resistivity region can be introduced by a single temperature-gradient-zone-melting process.
However, for many types of semiconductor devices, more than two regions of different type conductivity or more than two regions of different levels of resistivity are required in a body of a semiconductor material. For example, a conventional silicon controlled rectifier or thyristor requires at least four regions of differing levels of resistivity and conductivity type for it to function as a device. In particular, a semiconductor controlled rectifier requires four contiguous regions of, respectively, P.sup.+, N, P and N.sup.+ type conductivity. Such devices can be made by the temperature gradient zone melting process by starting with an N-type semiconductor body and subjecting it to three separate and successive temperature gradient zone melting processes to produce the P.sup.+, P and N.sup.+ regions respectively in the body of N-type semiconductor material. Each separate temperature gradient zone melting process takes time, requires separate photolithography and etching steps, separate metal deposition steps and separate thermal gradient processing. Thus, it is clear that four or more sequential temperature gradient zone melting process would detrimentally decrease the final device yield of such a combined process.
Consequently, it is desirable to be able to introduce more than one region of a new level of resistivity and conductivity type in a body of semiconductor material with a single temperature gradient zone melting process utilizing a single liquid zone.
An object of this invention is to provide a new and improved process of introducing multiple regions of differing resistivity and conductivity type in a body of semiconductor material which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved process of making multiple P-N junctions in a body of semiconductor material.
Another object of this invention is to provide a new and improved method of making thyristors.
A further object of this invention is to provide a new and improved semiconductor device embodying recrystallized regions of semiconductor material having solid solubility of a metal therein and regions of different and like type conductivity but different levels of resistivity formed by diffusion therein.
Other objects will, in part, be obvious and will, in part, appear hereinafter.